Any domain can have any NS records essentially. Example.com could use testdomain.com's name servers as long as they are authoritative. If you are doing a sub domain, I would recommend making another zone file for it rather than use the $ORIGIN function in a zone. But that's entirely up to you.

In dig you can query for NS records by doing what you typed. If you want to query a particular server, you specify it using the @ symbol. Like so: